This invention is directed to the recovery of volatile organic compounds from activated carbon adsorption beds. In particular, the invention is directed to an improved method of regenerating the activated carbon to thereby improve on-line time and the percentage of volatile organic compounds recovered from carbon adsorption beds.
Carbon adsorption beds are widely used in many industries to remove volatile organic compounds from process gas streams; for example, air or nitrogen streams. In a typical carbon bed operation, a process gas stream containing volatile organic compounds (xe2x80x9cVOCSxe2x80x9d) is passed at through the carbon bed at a selected temperature and pressure. The VOCS are adsorbed from the gas stream onto the carbon particles of the bed. Frequently, monitoring devices selective to the VOCS sought to be removed are placed on the outlet side of the carbon bed during the adsorption process. When the bed ceases to adsorb VOCS, a breakthrough is detected indicating the carbon bed is saturated and the process gas stream is diverted to a second carbon bed. The saturated carbon bed is then either unloaded and disposed of in an approved manner or, more economically, the carbon bed is regenerated for future on-line use and the VOCS recovered for recycling, disposal or fuel use depending on the type of VOCS.
In a simplified example of regenerating a carbon bed for additional use, the regeneration is carried out by passing steam at a selected temperature and pressure through the carbon bed, condensing the steam and any stripped VOCS on the outlet side of the bed, and recycling or venting effluent gases after condensation. The steam stripping is typically performed countercurrent to the inlet/outlet direction of the carbon bed during the adsorption phase. After the steam stripping is completed, the carbon bed is cooled by the passage of cooling gas through the bed, typically in the same direction as the VOCS were passed through the bed during the adsorption phase. Once cooling is completed, the carbon bed is placed in a xe2x80x9cready positionxe2x80x9d for on-line use when the bed currently on-line is saturated.
A thorough discussion of adsorption, carbon beds, and regeneration can be found in Kirk-Othmer: Encyclopedia of Chemical Technology, 4th Ed. (New York: John Wiley and Sons), Vol 1, pages 493-528 (adsorption in general); Vol 1, pages 529-573 (adsorption, gas separation) and Vol. 4, pages 1015-1037 (activated carbon and its uses). Additional information can be found in C. L. Mantell, ADSORPTION 2nd Ed. (New York: McGraw-Hill Book Company, 1951), pages 224-254; and John W. Hassler, Activated Carbon (New York: Chemical Publishing Company 1963), pages 55-66. The Mantell work, on pages 241-244, specifically describes a prior art acetone recovery unit.
It is the object of this invention to provide an improved process for regenerating carbon adsorption beds to optimize both the recovery of volatile organic compounds adsorbed on the bed and the operating capacity of the carbon bed when it is returned to on-line use.
It is a further object of this invention to provide improved process for regenerating carbon adsorption beds whereby the carbon beds are cooled in a direction opposite to the normal inlet-outlet direction which the bed is operated to thereby optimize both the recovery of volatile organic compounds adsorbed on the bed and the operating capacity of the carbon bed when it is returned on-line by providing for a outlet-to-inlet cooling
The invention describes a process for the removal of one or a plurality of volatile organic compounds from a process gas stream containing same by the use of an activated carbon containing unit and regenerating the carbon of said unit, said process comprising the steps of:
passing a process gas containing one or a plurality of volatile organic compounds (xe2x80x9cVOCsxe2x80x9d) capable of being adsorbed on an activated carbon though an activated carbon containing unit to thereby adsorb the volatile organic compounds onto said carbon, said passage being for a selected time at a selected temperature and pressure for a selected time;
shutting off the flow of process gas to the unit and passing steam for a selected time at a selected temperature and pressure through the unit in a direction opposite to direction of the flow of the volatile organic compounds containing process gas to thereby desorb said volatile organic compounds from the activated carbon;
condensing the steam and volatile organic compounds, and separating recovering same;
shutting off the flow of steam to the unit and passing atmospheric air through the unit at a selected pressure and for a selected time, the air being passed through the unit in the same direction as the steam was passed through the unit and not, as in the case of the prior art, in the same direction as was the VOC-laden process air passed through the unit, to thereby dry the carbon contained therein and lower its temperature to a selected temperature; and
returning said unit to on-line operating status.